add tutorial issue template

.github/ISSUE_TEMPLATE/tutorial.md

@@ -0,0 +1,24 @@
+---
+name: Tutorial request
+about: Suggest a topic for a new tutorial and propose where to put it in the [planned outline](https://github.com/NixOS/nix.dev/issues/572). Please check [existing tutorial requests](https://github.com/NixOS/nix.dev/issues?q=is%3Aopen+is%3Aissue+label%3Atutorial) before submitting. Check our [guide on writing new tutorials](https://nix.dev/contributing/documentation/writing-a-tutorial) if you want to help.
+labels: tutorial
+---
+
+**Learning goals**
+<!-- Which skill do you want to acquire? -->
+
+**Prerequisites**
+<!-- What do you already know how to do? -->
+
+**Outline**
+<!-- Which steps do you envision the tutorial to cover? -->
+
+**Prior art**
+<!-- Are there existing materials that are related, similar, or almost but not quite what you need? -->
+
+**Willing to help?**
+<!-- Are you willing to help write the tutorial? -->
+
+**Priorities**
+
+Add :+1: to [issues you find important](https://github.com/NixOS/nix.dev/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).